In the manufacture of electron discharge devices employing a cathode which emits electrons onto an anode, the electronemitting member is frequently a filamentary coil of relatively small diameter wire which is heated to incandescence by the application of suitable filament current. During this process copious supplies of electrons are directed onto a selected area of the anode.
In such devices, and particularly in X-ray tubes, the filament is precisely controlled as to length and diameter so as to form a specifically shaped electron beam which will be focused onto a single discrete area of the anode surface known as the focal spot, which area is of a predetermined size. In an X-ray tube the anode surface will be angled at an inclination of about 10.degree. - 15.degree., for example, and the focal spot will be a rectangular shape. However, when viewed from one side, the virtual focal spot will appear as substantially square, having four sides about 1.2 mm long, for example.
In order to achieve a focal spot of the desired size and shape, the filament may be mounted within a metal focusing cup or head to which suitable electrical potential is applied so as to aid in shaping the electron beam to produce the desired spot. This is fully explained in U.S. Pat. No. 3,103,591 and further described in U.S. Pat. No. 3,743,836, both of which patents are assigned to the assignee of this application.
It has been found that during operation of a tube having a filament type cathode of this nature the focal spot sometimes becomes altered in size and shape. This is believed to be due to the fact that the filament support structure becomes subjected to extremely high temperatures such as about 900.degree. - 950.degree. C, for example, which creates thermal and mechanical stresses tending to distort the parts and consequently, to also deform the filament.
The ends of the filament are fixed to the ends of spaced metal posts with substantially their entire lengths spanning the space between the posts. The posts, which are conveniently of molybdenum, are supported within sleeves or eyelets of nickel which are in turn mounted within bores formed in insulators, preferably of ceramic, fixed to the cathode head. Lead wires of nickel or the like are welded to the eyelets for providing the filament with suitable filament potential. It is believed that heat stresses within the lead wire-eyelet-post assembly causes the aforesaid undesirable distortion of the filament which produces undesirable alternation of the electron beam. In some cases distortion of the filament has been severe enough to cause shorting of the filament with closely spaced portions of the cathode head.